There has been substantial interest in the preparation and characterisation of particles of compound semiconductors having dimensions in the range 2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. These materials exhibit size-tuneable electronic properties that can be exploited in many commercial applications such as optical and electronic devices, biological labelling, solar cells, catalysis, biological imaging, light-emitting diodes, general space lighting and both electroluminescence and photoluminescence displays, amongst many new and emerging applications. The most studied of semiconductor materials have been the chalcogenide II-VI (i.e. group 12-group 16) materials, such as ZnS, ZnSe, CdS, CdSe and CdTe. Most noticeably, CdSe has been greatly studied due to its optical tuneability over the visible region of the spectrum.
Two fundamental factors, both related to the size of the individual QDs, are responsible for their unique properties. The first is the large surface-to-volume ratio: as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. Thus, surface properties play an important role in the overall properties of the material. The second factor is that their electronic properties change as a function of particles size; their band gap increases as the size of the particles decreases. This effect is a consequence of the confinement, analogous to the ‘electron in a box’ phenomena of quantum mechanics. The electronic states of QDs exhibit discrete energy levels similar to those observed in atoms and molecules, rather than continuous bands, as observed in the corresponding bulk semiconductor material. For a QD, the electron and hole produced by absorption of electromagnetic radiation (i.e., absorption of a photon) with energy greater than the first excitonic transition, are closer together than in the corresponding macrocrystalline material. Consequently, the Coulombic interaction between the electron and hole leads to a narrow bandwidth emission that is dependent upon the particle size and composition. QDs have higher kinetic energy than the corresponding macrocrystalline material and, consequently, the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Single core semiconductor QDs, which consist of a single semiconductor material along with an outer organic passivating layer, as illustrated in FIGS. 1A and 1B, can have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the QD surface which lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds on the surface of the QD is to grow a second inorganic material (typically a material having a wider band-gap and small lattice mismatch compared to the core material) on the surface of the core particle. Such a multi-shell QD is referred to as a ‘core/shell’ QD. Core/shell particles separate carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example of a core/shell QD is a QD having a ZnS shell grown on the surface of a CdSe core to provide a CdSe/ZnS core/shell QD.
Another approach is to prepare a core/multi-shell structure having a core of a wide bandgap material, coated with a thin shell of narrower bandgap material, which is, in turn, coated with another wide bandgap layer. An example is CdS/HgS/CdS grown using a substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS. With such core/multi-shell QDs the electron-hole pair is confined to a single shell layer. In the CdS/HgS/CdS material, photo-excited carriers are confined in the HgS layer.
As mentioned above, the coordination about the final inorganic surface atoms in any core, core/shell or core/multi shell QD is incomplete. Highly reactive non-fully coordinated atoms ‘dangling bonds’ on the surface of the particle tend to cause the particles to agglomerate. This problem can be overcome by passivating (capping) the ‘bare’ surface atoms with protecting organic groups. The outer most layer of organic material (capping agent) helps to inhibit particle aggregation and also further protects the QDs from their surrounding chemical environment. The capping agent may also provide a chemical linkage to attach other inorganic, organic or biological materials to the QD. In many cases, the capping agent is the solvent in which the QD preparation is undertaken, and may be a Lewis base compound, or a Lewis base compound diluted in an inert solvent, such as a hydrocarbon, whereby there is a lone pair of electrons that are capable of donor type coordination to the surface of the QD.
Important issues concerning the synthesis of high quality semiconductor QDs are particle uniformity, size distribution, quantum efficiencies and for use in commercial applications their long-term chemical and photostability. Most of the more recent methods of QD synthesis are based on the ‘nucleation and growth’ method described by Murray, Norris and Bawendi (C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc. 1993, 115, 8706), who used organometallic solutions of metal-alkyls (R2M) where M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphine sulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP). Those precursor solutions were injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-400° C. to produce TOPO coated/capped semiconductor nanoparticles of II-VI material. The size of the particles is controlled by the temperature, capping agent, concentration of precursor used and the length of time at which the synthesis is undertaken, with larger particles being obtained at higher temperatures, higher precursor concentrations and prolonged reaction times. This organometallic route has advantages over other synthetic methods, including greater monodispersity and high particle crystallinity. As mentioned, many variations of this method have now appeared in the literature and routinely give good quality core and core-shell nanoparticles in terms of both monodispersity and quantum yield.
Single-source precursors have also proved useful in the synthesis of semiconductor QD materials of II-VI. Bis(dialkyldithio-/diselenocarbamato)cadmium(II)/zinc(II) compounds, M(E2CNR2)2 (where M=Zn or Cd, E=S or Se and R=alkyl), have been used in a similar ‘one-pot’ synthetic procedure, which involved dissolving the precursor in tri-n-octylphosphine (TOP) followed by rapid injection into hot tri-n-octylphosphine oxide/tri-n-octylphosphine (TOPO/TOP) above 200° C.
Fundamentally, all of the above procedures rely on the principle of high temperature particle nucleation, followed by particle growth at a lower temperature. Moreover, to provide a monodispersed ensemble of nanoparticles in the 2-10 nm range there must be proper separation of the nanoparticle nucleation step from the nanoparticle growth step. In the methods discussed above, separation is achieved by rapid injection of a cooler solution of one or both precursors into a hotter coordinating solvent (containing the other precursor if otherwise not present), which initiates particle nucleation. The sudden addition of the cooler solution upon injection subsequently lowers the reaction temperature (the volume of solution added is typically about ⅓ of the total solution) and inhibits further nucleation. Particle growth (being a surface catalyzed process or via Ostwald ripening, depending on the precursors used) continues to occur at the lower temperature. Thus, nucleation and growth are separated, yielding a narrow nanoparticle size distribution. This method works well for small-scale synthesis, where one solution can be added rapidly to another while keeping a reasonably homogenous temperature throughout the reaction. However, on larger preparative scales, large volumes of solution are required to be rapidly injected into one another and significant temperature differentials can occur, leading to unacceptably large particle size distributions.
Applicant's commonly owned U.S. Pat. No. 7,588,828 (filed Sep. 10, 2007 and issued Sep. 15, 2009), U.S. Pat. No. 7,803,423 (filed Apr. 27, 2005 and issued Sep. 28, 2010), U.S. Pat. No. 7,985,446 (filed Aug. 11, 2010 and issued Jul. 26, 2011) and U.S. Pat. No. 8,062,703 (filed Aug. 10, 2010 and issued Nov. 22, 2011) the entire contents of which are incorporated herein, describe methods of producing large volumes of high quality monodisperse QDs. QD precursors are provided in the presence of a molecular cluster compound under conditions whereby the integrity of the molecular cluster is maintained and acts as a well-defined prefabricated seed or template to provide nucleation centres that react with the chemical precursors to produce high quality nanoparticles on a sufficiently large scale for industrial application.
It will be appreciated from the foregoing discussion that many of the nanoparticle materials that have been extensively studied to date incorporate cadmium ions. There are, however, many environmental problems associated with the use of cadmium and other heavy metals such as mercury and lead based materials and so there is a need to develop non-cadmium containing nanoparticle materials. In particular, it is desirable to produce non-cadmium containing quantum dots that exhibit similar monodispersities and size-tuneable photoluminescent spectra to current cadmium based materials. Commercial needs also dictate that such materials should be produced in high yields on a large-scale, as cheaply as possible.
Colloidal synthesis of InP nanoparticles is known in the prior art. Syntheses typically use tris(trimethylsilyl)phosphine ((TMS)3P) as the phosphorus precursor and proceed via hot injection. Alternative approaches use single source precursors, for example, Green and O'Brien (M. Green, P. O'Brien, Chem. Commun., 1998, 2459) use LiPtBu2 to form the single source precursor In(PtBu2), which decomposes in 4-ethylpyridine to form InP. High temperatures are typically required for those reactions. If a single source precursor is used, that precursor must be synthesized, requiring time and effort.
The use of phosphine gas in the metallo-organic synthesis of InP semiconductors was first described in the 1960s by Didchenko et al. (R. Didchenko, J. E. Alix, R. H. Toeniskoetter, J. Inorg. Nucl. Chem., 1960, 14, 35). That method involves reacting trimethylindium (InMe3) with phosphine to form InP and methane. Since then, reactions between InMe3 and PH3 gases have been extensively used for the synthesis of InP semiconductor films by metallo-organic chemical vapour deposition (MOCVD) (H. M. Manasevit, Appl. Phys. Lett., 1968, 12, 156), chemical beam epitaxy (CBE) (C. Theodoropoulos, N. K. Ingle, T. J. Mountziaris, Z. Y. Chen, P. L. Liu, G. Kioseoglou, A. Petrou, J. Electrochem. Soc., 1995, 142, 2086), and gas-source molecular beam epitaxy (GSMBE) (H. Ando, N. Okamoto, A. Sandhu, T. Fujii, J. Cryst. Growth, 1991, 59, 431). Single crystal InP growth in solution from tBu3In and phosphine gas is also documented in the prior art (T. J. Trentler, S. C. Goel, K. M. Hickman, A. M. Viano, M. Y. Chiang, A. M. Beatty, P. C. Gibbons, W. E. Buhro, J. Am. Chem. Soc., 1997, 119, 2172). The use of phosphine gas in the synthesis of InP nanoparticles, however, is little documented in the prior art, though a few examples do exist.
PH3 has been used in solution-based syntheses of InP nanoparticles, either using gaseous PH3 or by generating it in situ. For example, Peng et al. report the use of phosphine gas dissolved in benzene to synthesise InP nanoparticles (See A. Peng, M. Hines, S. Perera, U.S. Pat. No. 7,850,777, issued 2010). In a typical synthesis, a cation precursor solution is produced by mixing In(Ac)3 with oleic acid and octadecene (ODE) at 80-130° C. under vacuum, until a clear solution is obtained. A portion of the cation solution, stored under N2 or Ar, is mixed with an equal quantity of ODE in a vessel in a glove box filled with N2 or Ar. A solution of PH3 in benzene is added, then the reaction vessel is capped and sealed at room temperature, before removing from the glove box. The vessel is connected to a nitrogen cylinder then the pressure is increased from 0 psi to 1000 psi. The pressurised vessel is then heated to 250° C. using a heating mantle; the pressure increases to ˜1700 psi. After 30 minutes at 250° C. the heating mantle and nitrogen cylinder are removed to quench the reaction. The vessel is cooled to room temperature. Once cool, the pressure is released and acetone is added to the reaction solution until it turns turbid. The mixture is centrifuged, then the solid is re-dissolved in toluene and retained as the product.
Li et al. generate phosphine gas in situ by reacting calcium phosphide with hydrochloric acid to synthesise InP nanoparticles (L. Li, M. Protiere, P. Reiss, Chem. Mater., 2008, 20, 2621). In a typical reaction, In(Ac)3 and myristic acid are mixed with ODE under inert conditions, in a three-necked flask fitted with a condenser. A separate flask loaded with Ca3P2 is connected to a precursor flask, via a column containing P2O5 to eliminate any water from the generated PH3. The In-precursor is heated to 100-120° C. until a clear solution is formed, then after degassing both flasks they are filled with Ar. After heating the In-precursor flask to 250° C., 4 M HCl is injected into the Ca3P2-containing flask; gaseous PH3 is produced, which is carried via the Ar flow, then bubbled through the In-precursor flask to synthesise InP nanocrystals, characterised by a colour change from colourless to dark red. After 20 minutes, once all the Ca3P2 has been consumed, the reaction solution is cooled and the nanoparticles are mixed with an acetone/chloroform/methanol mixture, then isolated by centrifugation. As the phosphine gas is released slowly, nanoparticle growth is thought to take place in the size focusing regime, resulting in a narrow particle size distribution, thus a well-defined excitonic peak.
Nedeljković et al. report the synthesis of InP nanorods, using PH3 generated in situ by the hydrolysis of tris(trimethylsilyl)phosphine ((TMS)3P) (J. M. Nedeljković, O. I. Mićić, S. P. Ahrenkiel, A. Miedaner, A. J. Nozik, J. Am. Chem. Soc., 2004, 126, 2632). In one example, 6.5 nm. In nanoparticles are synthesised at room temperature, by decomposing C5H5In in toluene and a little trioctylamine, in the dark. A solution of the In nanoparticles is further diluted with toluene, which is then mixed with (TMS)3P in the presence of MeOH or thiophenol (PhSH); the alcohol or thiol hydrolyses the P—SiMe3 bond to form PH3. Upon mixing, nanowires form immediately. The solution is then heated to 220° C. for 2 minutes. A colour change to brown is observed as the InP nanowires form. The resulting nanowires are isolated by dilution in toluene, followed by precipitation with MeOH. The particles are cleaned by dissolving in 1% HDA in chloroform.
The use of PH3 in the growth of InP by chemical vapour deposition (CVD), chemical beam epitaxy (CBE) and molecular beam epitaxy (MBE) has, more recently, been adapted to produce nanoparticles or nanowires. In contrast to the colloidally synthesised nanoparticles disclosed in the present disclosure, using CVD, CBE and MBE the nanoparticles are grown on a substrate, restricting their use to thin film applications.
InP nanowires doped with Zn or S have been grown by metal-organic vapour-phase epitaxy (MOVPE), a type of CVD, using PH3 gas as the phosphorus source, as described by van Weert et al. (M. H. M. van Weert, A. Helman, W. van den Einden, R. E. Algra, M. A. Verheijen, M. T. Borgström, G. Immink, J. J. Kelly, L. P. Kouwenhoven, E. P. A. M. Bakkers, J. Am. Chem. Soc., 2009, 131, 4578). The nanowires are grown via a vapour-liquid-solid mechanism, using catalytic 50 nm gold colloids deposited on a phosphorus-terminated InP substrate. PH3 and InMe3 gases are used as the P and In precursors, respectively, with diethylzinc being used to dope the resultant InP with Zn, and H2S for sulphur-doping. The resultant nanowires have diameters less than 100 nm, which can be further reduced by wet-chemical etching using H2SO4/H2O2/H2O to remove any competitive radial growth.
In an example of InP nanowires formation using CBE, Chiaramonte et al. use InMe3 and PH3 as the In and P sources, respectively (T. Chiaramonte, L. H. G. Tizei, D. Ugarte, M. A. Cotta, Nano Lett., 2011, 11, 1934). The InP nanowires are grown on a GaAs (100) substrate in a CBE chamber, using 10 nm or 25 nm Au nanoparticle catalysts. InMe3 diluted with an H2 carrier gas, and PH3 at a flow rate of 15 sccm, thermally decompose at the growth temperature of 420° C. to form wurtzite phase nanowires. The nanowires are cooled in a PH3 atmosphere.
Reiss et al. describe the growth of InP nanowires by MOCVD, using PH3 as the phosphorus source (P. J. Reiss, W. J. Toe, F. Wang, S. Paiman, Q. Gao, H. H. Tan, C. Jagadish, Nano Lett., 2011, 11, 2375). Epitaxial growth takes place on an InP (111)B substrate, catalysed by 30 nm Au nanoparticles. The nanowires are grown in an MOCVD reactor, using InMe3 and PH3 precursors. Wurtzite phase nanowires predominate when the pressure is maintained at 100 mbar, at a growth temperature of 490° C., with a P/In ratio of 44 and a growth time of approximately 20 minutes.
PH3 has been used in metal organic vapour phase epitaxy (MOVPE) to synthesise InP quantum dots on GaAs and Ga0.5In0.5P surfaces, using Stranski-Krastanow growth (J. Johansson, W. Seifert, V. Zwiller, T. Junno, L. Samuelson, Appl. Surf. Sci., 1998, 134, 47). In that method, 2D expitaxial layers of a material are initially deposited, but above a critical thickness the layers relax under compressive strain to form 3D islands or dots and a thinner 2D wetting layer. In one example, freestanding InP islands are grown on GaAs (001) substrates in an MOVPE reactor at 100 mbar. Two monolayers of GaP are grown using trimethylgallium (GaMe3) and PH3, to stabilise the GaAs surface. Three monolayers of InP are then deposited at a rate of 0.6 ML s−1 at 600° C., using InMe3 and PH3 precursors. The samples are then annealed under a PH3/H2 atmosphere; during annealing the island height decreases from 19 nm to 7 nm over 30 minutes, thus the process can be used to tune the photoluminescence wavelength by altering the island height.
The methods to synthesise InP-based nanoparticles using PH3 gas described in the prior art typically involve the use of high temperatures and/or pressures. CVD and chemical beam experiments require expensive equipment and the resultant nanoparticles are restricted to thin-film application. There is a need for methods for producing monodisperse, organically-capped QDs at relatively low temperature and without the need for high pressures.